Magnetic shielding construction for electric transformers

ABSTRACT

In an electric transformer, magnetic laminations for shielding the main magnetic core from leakage magnetic flux are positioned between the inner periphery of the coil and the portion of the main magnetic core which extends through the coil window. The shield laminations are arranged in closed magnetic loops, with the major surfaces of the shielding laminations lying in planes perpendicular to the major surfaces of the main core laminations, whereby only the edges of the shielding laminations are presented to the leakage magnetic flux, thereby minimizing eddy current losses and consequent overheating of the laminations.

United States Patent [72] Inventors Samuel L. Foster Eagle, Wis.; Reed H. Johnston, Wellesley, Mass. [21] Appl. No. 760,430 [22] Filed Sept. 18, 1968 [45] Patented May 4, 1971 [73] Assignee Allis-Chalmers Manufacturing Company Milwaukee, Wis.

[54] MAGNETIC SHIELDING CONSTRUCTION FOR ELECTRIC TRANSFORMERS 13 Claims, 3 Drawing Figs.

[52] US. Cl 336/84, 336/212, 336/234 [51] Int. Cl 1101f 15/04 [50] Field of Search 336/84, 212, 213,234, 5,214, 215

[56] References Cited UNITED STATES PATENTS 1,610,867 12/1926 1.6mm 336/212 1,910,253 5/1933 Keinath 336/234X 2,314,912 3/1943 Troy 336/212X I 12% l 21,24 A 19 2,370,045 2/1945 Keto 336/84 2,449,434 9/ 1948 Whitman 336/212X 2,908,880 10/ l 959 Steinmayer et a1. 336/212X 3,281,745 10/1966 Moore-ct a1. 336/84 3,464,041 9/1969 Watenman 336/5 FOREIGN PATENTS 677,841 12/1929 France 336/212 Primary Examinerlhomas J. Kozma Attorneys-Robert C. Sullivan, Lee H. Kaiser and Robert B.

Benson Ill ll/I 'i MAGNETIC SHIELDING CONSTRUCTION FOR ELECTRIC TRANSFORMERS BACKGROUND OF THE INVENTION core support within the coil window of a shell-type transformer are disclosed in such prior art U.S. Pat. Nos. as 2,370,045 to Keto, and 3,28l,745 to Moore et al., to conduct the lines of stray leakage flux and to prevent eddy current overheating of these members. Such magnetic shields comprise bundles of magnetic laminations referred to as Core packs" which cause the stray leakage magnetic flux to pass through the laminated shield and prevent the stray leakage flux from entering the tongue wedge and T-shaped core support.

Overheating of the laminations in the tongue portion of the magnetic core of a shell-type transformer may occur as a result of leakage flux entering the laminations at right angles to the plane of the laminations and generating eddy currents which produce I' R losses. In larger shell form units, such as those above 200 MVA, the leakage flux density may be sufficiently high to produce excessive eddy currents in laminations and clamping members having a surface normalto the leakage flux wider than approximately 0.5 inch. Such eddy currents may generate sufiicient heat to cause the electrical insulation of the transformer to become thermally unstable and to fail electrically.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to providean electrical transfonner having reduced temperature rise caused by eddy current overheating resulting from leakage magnetic flux.

I It is a further object of the invention to provide an electrical In achievement of these objectives, there is provided in ac-- cordance with this invention an electrical transformer having shielding magnetic laminations for shielding the main magnetic core from leakage magnetic flux. The shielding laminations are positioned between the inner periphery of the coil and the portion of the main magnetic core which extends through the coil window. The shielding laminations are arranged in closed magnetic loops, with the major surfacesof the shielding laminations lying in planes perpendicular to the major surfaces of the main core laminations, whereby only the edges of the shieldingilaminations are presented to the leakage magnetic flux, thereby minimizing eddy current losses and consequent overheating of the laminations.

Further objects and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings. I

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view in vertical section, partially schematic, of a shell-type transfomter embodying the shielding lamination structure of the invention;

' FIG. 3 is a view in perspective of the bundles of shielding laminations overlying the core tongue and supported by the traylike support member.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now the drawings, a circular coil, shell-type transformer is illustrated enclosed in a metallic tank 10 having a shelf portion 11 around its entire periphery which supports the magnetic steel laminations 12L of the magnetic core legs and also the laminations 12Y of the magnetic core yokes. The invention will be described as embodied in a single phase transformer having a magnetic core arrangement such as that shown in FIGS. 1 and 2. The laminated main magnetic core is of the well known shell-type and preferably has two back-to back rectangular loops 14A and 14B, and each horizontal layer of each loop 14A and 148 preferably comprises a pair of yoke laminations 12Y having mitered ends abutting against the mitered ends of a pair of leg laminations 12L. The back-toback leg laminations 12L of the two loops 14A and 148 form the iron tongue 15 of the main core which is surrounded by the cylindrical electrical coil 16 of the transformer. In large power transformers of the type with respect to which this invention is particularly useful, the laminations 12L and 12Y of the main magnetic core would normally have the major surface thereof lying in a horizontal plane, with the end edges of the leg laminations 12L are of different widths and the backto-back loops 14A and 148 form a tongue 15 of cruciform cross section which passes through the window of coil 16. Insulating members 17 of suitable material such as maple wood may be disposed between the wall portions of tank 10 and the magnetic core laminations 12L and 12Y.

Coil 16 preferably comprises a plurality of axially spaced apart pancake windings 18, certain of which are high voltage windings and others of which are low voltage windings. The primary winding of the transformer may comprise a plurality of high voltage pancake windings adjacent the center ofthe coil stack and the secondary winding of the transformer may comprise groups of low voltage pancake windings on opposite ends of the stack in a well-known manner. Annular insulating spacers (not shown) may be provided between adjacent pancake windings I8, and high-to-low insulating barriers (not shown) may be disposed between the primary and secondary windings in a manner well-known in the art. The pancake coils 18 may be supported on a center foundation tube 19 of suita- ,ble material such as phenolic insulation having a dielectric constant approximately equal to oil impregnated Kraft paper.

A plurality of telescoped inside insulation cylinders 20 may surround tube 19 and be disposed between tube 19 and the pancake windings I8. Each inside insulation cylinder 20 may be of Kraft paper wound into a tube and having spacer sticks 22 secured thereto which extend parallel to the coil axis and are spaced apart circumferentially to provide cooling ducts axially of the coil for the transformer oil (not shown) within tank 10 in which the core and coil assembly is immersed. The structure described hereinbefore is known in the art. One of the insulating cylinders 20 may support an electrostatic shielding member of the type described and claimed in US. Pat. application, Ser. No. 741,980, now abandoned filed July 2, I968, of August F. Wishman, .Ir., Harding B. Hansen and Samuel L. Foster, entitled Electrostatic Shield for Stationary Electrical Induction Apparatus.

' The magnetic shield in accordance with the invention preferably comprises a secondary magnetic core which normally carries the leakage flux and which also carries a portion of the main exciting flux.

The magnetic shield in accordance with the invention comprises a plurality of bundles or packs A, B, C, D, E, and F of magnetic strip steel laminations disposed within the coil window adjacent the top surface of the main core, and an equal number of bundles or packs G, H, I, J, K and L disposed within the coil window adjacent the bottom surface of the main core. The bundles A, B and C overlie the upper surface of the rectangular loop 14A of the main core while the lamination bundles or packs D, E and F overlie the rectangular loop 148 of the main core. Similarly, the lamination packs G, H and I underlie the core loop 14A of the main core while the lamina- The-lamination packs A through L, inclusive, progressively increasejiri height from the outer to the center packs to conform to the site of the circular opening in the foundation tube 19.

To suitably support the core packs A, B, C, D, E and F and to maintain the core packs in properly assembled relation, the radially inner edges of the laminations of the core packs are welded to the radially outer surface of the base portion 32 of a plurality of spaced traylike supports 30 (FIGS. 1. and 3). The traylike supports 30 have end portions 34 which extend in a radially outward direction. The radially inner surface of the base portion 32 of each traylike support 30 is welded or otherwisesuitably attached to the radially outer head portion 38 of a T-shaped member generally indicated at 36. Traylike support 30 and Tshaped member 36 may be made of stainless steel, for example. Member 36 includes a tongue portion 40 which extends lengthwise of the main core tongue between the two back-to-back main core loops 14A and MB. The

traylike supports 30 are positioned at spaced intervals along the head portion 38 of the T-shaped support 36.

The core packs G, H, l, J, K and L are supported in a manner similar to that just described for core packs A, B, C, D, E and F by a plurality of spaced traylike supports 30 which in turn are mounted on the T-shaped member 36.

An important feature of the construction is the fact that the laminations of all of the magnetic shield packs A through L, inclusive, have the major surfaces thereof arranged in vertical planes perpendicular to the horizontal plane of the major surfaces of the laminations of loops 14A and 14B of the main core. Thus, the major surfaces of the laminations of the magnetic shield bundles A through L, inclusive, are approximately parallel to the leakage magnetic flux lines which enter the edges of the laminations of the packs A through L, rather than entering through the major surfaces thereof. it will be appreciated that this construction substantially reduces the eddy current PR heating generated by leakage flux, in comparison to a transformer wherein the flux enters perpendicular to the major surfaces of the laminations. The vertically stacked laminations of the core packs A through L, inclusive, carry all of the leakage flux generated in their sector of the inner periphery until the transformer current reaches such magnitude as to cause the packs A through L to become saturated. Additional leakage flux generated by transformer currents of greater magnitude will then penetrate to the main core laminations. However, by suitably proportioning the crosssectional area of the magnetic shield packs A through L, inclusive, the amount of leakage flux penetrating into the main core laminations may be made such as to be of insufficient density to cause excessive eddy current heating in the main core laminations. Eddy current heating is minimal in the vertical laminations in the core packs A through L, inclusive, even when they are saturated since only their edges are perpendicular or nonnal to the leakage flux.

As best seen in the view of H0. 2, each of the core packs A through L, inclusive, is of a rectangular loop shape. Thus, as seen in H6. 2 each of the core packs A, B and C comprises a pair of oppositely disposed le'gs A, B and C respectively, and a pair of oppositely disposed yoke portions, Ay, By and C respectively. The leg and yoke laminations of the respective core packs are formed of straight strip magnetic material. The leg and yoke laminations are connected by curved strip laminations such as those indicated at A B and DC One of the legs A B and C of the respective core packs A, B and C overlie the center core tongue 15 of the main core while the opposite legs of the same core packs overlie one of the outer legs 12,, of the main core loop 14A.

The core packs D, E and F each include oppositely disposed legs, and oppositely disposed yoke portions connected by curved corner portions as just described in connection with core packs A, B and C, each pack D, E and F having one of its .leg portions overlying the center core tongue 15 of the main core, while the other leg portion of the respective core packs D, E and F overlies an outer leg 12, of the main core loop 148.

While the core packs A through L, inclusive, have been shown and described as formed of straight leg and yoke laminations joined by curved corner laminations to form closed loops, the core packs do not necessarily have to be formed in-this manner. For example, the core packs may be formed of strip laminations which were initially wound as a strip in spiral form, cut, and then reassembled to form a closed loop core having laminar strips in which a curved comer portion or portions are included in the same integral strip as a straight leg or yoke portion or portions.

By forming the respective core packs A, B, C, D, E, F, G, H, l, J, K and L in the form of closed loops, a completemagnetic circuit is provided for the leakage flux lines emerging from the coil 16, so that the leakage flux lines emerging from the outer periphery of the coil 16 may return to the corresponding radial line at the inner periphery of the coil through one of the magnetic packs A through L, inclusive, rather than returning through the tank wall or through the core clamps. In the prior art, one of the leakage flux tubes" which often cause undesirable heating of the tank and core clamps is one which followed a completely external path in returning to the top and bottom core packs within the tongue, where the shielding core packs were of the prior art-type not forming a closed loop type magnetic circuitas in the case of the core packs of the present application. Such paths followed by the leakage magnetic flux of the-prior art transformers caused undesirable heating of the tank and core clamps, which is prevented by the closed magnetic loop-type of magnetic shields described hereinbefore and shown in the drawings of the present application.

A further advantage of the magnetic shield structure hereinbefore described is that the magnetic shielding laminations present a magnetic path for leakage flux which substantially reduces penetration of the leakage flux into the main core laminations, thereby substantially preventing eddy currents and consequent overheating of the main core laminations. At the same time, the fact that the magnetic shield laminations present only their edges rather than their major flat surfaces to the lines of leakage magnetic flux substantially prevents the generation of eddy currents and consequent heat losses in the magnetic shielding laminations. A further advantage of the use of the magnetic shielding laminations is the fact that in addition to carrying the leakage magnetic flux as just described, the shielding laminations also carry a portion of the main'field magnetic flux, thereby reducing the total cross-sectional area required for the core and providing more efficient space utilization. The small cross sectional area of the magnetic core and more efficient space utilization as just explained results in savings in the cost of manufacture of the transformer.

From the foregoing detailed description of the present invention, it has been shown how the objects of the invention have been obtained in a preferred manner. However, modifications and equivalents of the disclosed concepts as readily occur to those skilled in the art are intended to be included within the scope of this invention. Thus, the scope of this invention is intended to be limited solely by the scope of the claims such as are or may hereafter be appended hereto.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

We claim:

1. An electrical power transformer subject to overheating of the magnetic core laminations as a result of magnetic leakage flux entering the laminations perpendicular to the planes thereof comprising, in combination, a coil having primary and secondary transformer windings and a window therein, a closed magnetic core formed of a plurality of laminations of magnetic material disposed in parallel planes extending through said window in said coil, and a magnetic shield loop extending through said window and formed of a plurality of laminations of magnetic material overlying the planar surfaces of the laminations of said closed core around the entire contour of said closed core, the portion of said magnetic shield loop within said window being disposed between the planar surface of the laminations of said magnetic core and the inner periphery of said-coil defining said window and comprising magnetic laminations having a width which is only a minor fraction of the width of said laminations of said closed core lying in planes perpendicular to the planes of said laminations of said closed magnetic core, whereby minimum leakage flux enters the laminations of said closed core perpendicular to the planar surfaces thereof and said magnetic shield loop carries part of the main flux.

2. An electrical power transformer as defined in claim 1 in which said magnetic core comprises oppositely disposed leg members connected to oppositely disposed yoke members and said coil surrounds one of said leg members and said magnetic shield loop comprises oppositely disposed leg members connected to oppositely disposed yoke members and each of said leg and yoke members comprises magnetic laminations in planes perpendicular to and overlying the planes of said laminations of the corresponding leg and yoke members of said closed magnetic core.

3. An electrical power transformer as defined in claim 1 in which said closed magnetic core comprises oppositely disposed leg members connected to oppositely disposed yoke members and is formed of flat stacked laminations and said coil surrounds one of said leg members and said magnetic shield loop comprises oppositely disposed leg members and oppositely disposed yoke members overlying the corresponding said leg and yoke members of said closed magnetic core and at least the portion of said magnetic shield loop within said window is formed of lamination strips of magnetic material lying in planes perpendicular to the planes of said flat stacked laminations.

4. An electrical transformer as defined in claim 3 wherein the portion of said magnetic shield loop within said window comprises a plurality of bundles of strips of magnetic material lying in planes perpendicular to the planes of said flat stacked laminations, said bundles being disposed side-by-side in a common plane parallel to the planes of said flat stacked laminations, the bundles at the center of said side-by-side bundles being of the greatest height where the leakage flux enters said flat stacked laminations at angles closest to the perpendicular and the height of said bundles decreasing progressively toward the end bundles wherein the leakage flux enters said flat stacked laminations at angles further removed from the vertical.

5. An electrical transformer comprising a coil having a window therein, a closed magnetic core having a winding leg portion formed of a plurality of core laminations of magnetic material in planes parallel to the coil axis extending through said coil window and yoke portions adjacent the axial ends of said coil, said core laminations extending the entire width of said winding leg portion, and a magnetic shield loop extending through said window and disposed between the inner periphery of said coil defining said window and the planar surfaces of the core laminations of said winding leg portion, at least the portion of said magnetic shield loop within said coil window comprising a plurality of side-by-side magnetic strip bundles intersected by a common plane parallel to the planes of said core laminations of said winding leg portion, each of said bundles comprising a plurality of magnetic strip laminations narrower than said core laminations and having a magnetic return path through the laminations of said yoke portions.

6. An electrical power transfonner as defined in claim 3 in which said lamination stripsof said magnetic shield loop are of the same material as said flat stacked laminations of said an approximately arcuate surface complementary to said circular window.

8. An electrical power transformer subject to overheating of the laminations as a result of magnetic leakage flux entering the laminations perpendicular to the planes thereof comprising, in combination,

a closed magnetic core formed of a plurality of laminations of magnetic material disposed in parallel planes and having opposed leg members joined by opposed yoke members,

first and second magnetic shield loops each of which is formed of a plurality of laminations of magnetic material and having opposed leg members joined by opposed yoke members, the leg and yoke members of said first and second magnetic shield loops overlying the planar surfaces of the uppermost and lowermost of the laminations respectively of the corresponding leg members and yoke members of said closed core, and

a coil having primary and secondary transformer windings and a window therein surrounding one leg portion of said closed core,

the portion of each said magnetic shield loop within said window being disposed between said planar surface of said laminations of said one leg portion and the inner surface of said coil defining said window and comprising laminations in planes perpendicular to the planes of said laminations of said closed core, whereby minimum leakage flux enters the laminations of said closed core perpendicular to the planar surfaces thereof and said shield loops carry part of the main magnetic flux.

9. An electrical power transformer in accordance with 7 claim 8 wherein said laminations of said first and second magnetic shield loops are of the same material as said laminations of said closed core and have a width which is only a minor fraction of the width of said laminations of said closed magnetic core.

10. A shell type electrical power transformer subject to overheating of the magnetic core laminations as a result of magnetic leakage flux entering the laminations perpendicular to the planes thereof comprising, in combination, a coil having primary and secondary transformer windings and a window therein, a closed magnetic core formed of a plurality of laminations of magnetic material disposed in parallel planes and having a center tongue extending through said window and leg members disposed on opposite sides of said center tongue, and first and second magnetic shield loops in back-toback relation each of which comprises a portion contiguous said center tongue extending through said window and another portion contiguous one of said leg members and formed of a plurality of laminations of magnetic material overlying the planar surfaces of the laminations of said magnetic core, the portion of each said magnetic shield loop within said window being disposed between the planar surfaces of laminations of said center tongue and the inner periphery of said coil defining said window and comprising magnetic laminations lying in planes perpendicular to the planes of said laminations of said magnetic core, whereby minimum leakage flux enters laminations of said center tongue perpendicular to the planar surfaces thereof and said first and second magnetic shield loops each carry part of the main flux.

11. An electrical transformer as defined in claim 10 and including a third and a fourth shield loop in back-to-back relation to each other each of which has a portion contiguous said center tongue extending through said window and another 1 portion adjacent one of said leg members formed of a plurality of laminations of magnetic material in underlying relation to the planar surfaces of the laminations of said magnetic core, the portion of each of said third and fourth shield loops continuous said center tongue comprising laminations perpendicular to the planes of the laminations of said center tongue.

12. An electrical transformer including a coil having a window therein, first and second closed magnetic cores having winding leg portions disposed back-to-back protruding through said window and also having yoke portions adjacent the axial ends of said coil, each of said closed magnetic cores being formed of core laminations of magnetic material in planes parallel to the axis said coil window, first and second magnetic shield loops extending through said window between the inner periphery of said coil defining said window and the upper .and lower surfaces of the winding leg portion of said first closed core, and third and fourth magnetic shield loops extending through said window between the inner periphery of said coil defining said window and the upper and lower surfaces of the winding leg portion of said second closed core, the

portion of each magnetic shield loop within said window comprising a plurality of side-by-side bundles of magnetic strip laminations narrower than said core laminations disposed in overlying relation to the planar surface of an outer core lamination 'of the winding leg portion of the corresponding 

1. An electrical power transformer subject to overheating of the magnetic core laminations as a result of magnetic leakage flux entering the laminations perpendicular to the planes thereof comprising, in combination, a coil having primary and secondary transformer windings and a window therein, a closed magnetic core formed of a plurality of laminations of magnetic material disposed in parallel planes extending through said window in said coil, and a magnetic shield loop extending through said window and formed of a plurality of laminations of magnetic material overlying the planar surfaces of the laminations of said closed core around the entire contour of said closed core, the portion of said magnetic shield loop within said window being disposed between the planar surface of the laminations of said magnetic core and the inner periphery of said coil defining said window and comprising magnetic laminations having a width which is only a minor fraction of the width of said laminations of said closed core lying in planes perpendicular to the planes of said laminations of said closed magnetic core, whereby minimum leakage flux enters the laminations of said closed core perpendicular to the planar surfaces thereof and said magnetic shield loop carries part of the main flux.
 2. An electrical power transformer as defined in claim 1 in which said magnetic core comprises oppositely disposed leg members connected to oppositely disposed yoke members and said coil surrounds one of said leg members and said magnetic shield loop comprises oppositely disposed leg members connected to oppositely disposed yoke members and each of said leg and yoke members comprises magnetic laminations in planes perpendicular to and overlying the planes of said laminations of the corresponding leg and yoke members of said closed magnetic core.
 3. An electrical power transformer as defined in claim 1 in which said closed magnetic core comprises oppositely disposed leg members connected to oppositely disposed yoke members and is formed of flat stacked laminations and said coil surrounds one of said leg members and said magnetic shield loop comprises oppositely disposed leg members and oppositely disposed yoke members overlying the corresponding said leg and yoke members of said closed magnetic core and at least The portion of said magnetic shield loop within said window is formed of lamination strips of magnetic material lying in planes perpendicular to the planes of said flat stacked laminations.
 4. An electrical transformer as defined in claim 3 wherein the portion of said magnetic shield loop within said window comprises a plurality of bundles of strips of magnetic material lying in planes perpendicular to the planes of said flat stacked laminations, said bundles being disposed side-by-side in a common plane parallel to the planes of said flat stacked laminations, the bundles at the center of said side-by-side bundles being of the greatest height where the leakage flux enters said flat stacked laminations at angles closest to the perpendicular and the height of said bundles decreasing progressively toward the end bundles wherein the leakage flux enters said flat stacked laminations at angles further removed from the vertical.
 5. An electrical transformer comprising a coil having a window therein, a closed magnetic core having a winding leg portion formed of a plurality of core laminations of magnetic material in planes parallel to the coil axis extending through said coil window and yoke portions adjacent the axial ends of said coil, said core laminations extending the entire width of said winding leg portion, and a magnetic shield loop extending through said window and disposed between the inner periphery of said coil defining said window and the planar surfaces of the core laminations of said winding leg portion, at least the portion of said magnetic shield loop within said coil window comprising a plurality of side-by-side magnetic strip bundles intersected by a common plane parallel to the planes of said core laminations of said winding leg portion, each of said bundles comprising a plurality of magnetic strip laminations narrower than said core laminations and having a magnetic return path through the laminations of said yoke portions.
 6. An electrical power transformer as defined in claim 3 in which said lamination strips of said magnetic shield loop are of the same material as said flat stacked laminations of said closed magnetic core and have a width which is only a minor fraction of the width of said flat stacked laminations of said closed core.
 7. An electrical power transformer as defined in claim 3 wherein the window in said coil is circular and said lamination strips of said magnetic shield loop vary in width so that the edges thereof adjacent the inner periphery of said coil define an approximately arcuate surface complementary to said circular window.
 8. An electrical power transformer subject to overheating of the laminations as a result of magnetic leakage flux entering the laminations perpendicular to the planes thereof comprising, in combination, a closed magnetic core formed of a plurality of laminations of magnetic material disposed in parallel planes and having opposed leg members joined by opposed yoke members, first and second magnetic shield loops each of which is formed of a plurality of laminations of magnetic material and having opposed leg members joined by opposed yoke members, the leg and yoke members of said first and second magnetic shield loops overlying the planar surfaces of the uppermost and lowermost of the laminations respectively of the corresponding leg members and yoke members of said closed core, and a coil having primary and secondary transformer windings and a window therein surrounding one leg portion of said closed core, the portion of each said magnetic shield loop within said window being disposed between said planar surface of said laminations of said one leg portion and the inner surface of said coil defining said window and comprising laminations in planes perpendicular to the planes of said laminations of said closed core, whereby minimum leakage flux enters the laminations of said closed core perpendicular to the planar surfaces thereof and said shield loops carry part of the main magNetic flux.
 9. An electrical power transformer in accordance with claim 8 wherein said laminations of said first and second magnetic shield loops are of the same material as said laminations of said closed core and have a width which is only a minor fraction of the width of said laminations of said closed magnetic core.
 10. A shell type electrical power transformer subject to overheating of the magnetic core laminations as a result of magnetic leakage flux entering the laminations perpendicular to the planes thereof comprising, in combination, a coil having primary and secondary transformer windings and a window therein, a closed magnetic core formed of a plurality of laminations of magnetic material disposed in parallel planes and having a center tongue extending through said window and leg members disposed on opposite sides of said center tongue, and first and second magnetic shield loops in back-to-back relation each of which comprises a portion contiguous said center tongue extending through said window and another portion contiguous one of said leg members and formed of a plurality of laminations of magnetic material overlying the planar surfaces of the laminations of said magnetic core, the portion of each said magnetic shield loop within said window being disposed between the planar surfaces of laminations of said center tongue and the inner periphery of said coil defining said window and comprising magnetic laminations lying in planes perpendicular to the planes of said laminations of said magnetic core, whereby minimum leakage flux enters laminations of said center tongue perpendicular to the planar surfaces thereof and said first and second magnetic shield loops each carry part of the main flux.
 11. An electrical transformer as defined in claim 10 and including a third and a fourth shield loop in back-to-back relation to each other each of which has a portion contiguous said center tongue extending through said window and another portion adjacent one of said leg members formed of a plurality of laminations of magnetic material in underlying relation to the planar surfaces of the laminations of said magnetic core, the portion of each of said third and fourth shield loops continuous said center tongue comprising laminations perpendicular to the planes of the laminations of said center tongue.
 12. An electrical transformer including a coil having a window therein, first and second closed magnetic cores having winding leg portions disposed back-to-back protruding through said window and also having yoke portions adjacent the axial ends of said coil, each of said closed magnetic cores being formed of core laminations of magnetic material in planes parallel to the axis of said coil window, first and second magnetic shield loops extending through said window between the inner periphery of said coil defining said window and the upper and lower surfaces of the winding leg portion of said first closed core, and third and fourth magnetic shield loops extending through said window between the inner periphery of said coil defining said window and the upper and lower surfaces of the winding leg portion of said second closed core, the portion of each magnetic shield loop within said window comprising a plurality of side-by-side bundles of magnetic strip laminations narrower than said core laminations disposed in overlying relation to the planar surface of an outer core lamination of the winding leg portion of the corresponding closed core.
 13. An electrical transformer in accordance with claim 12 wherein said transformer is of the shell-type and said first and second closed cores are rectangular loops and said back-to-back winding leg portions form the core tongue of said shell-type transformer and wherein said first and third shield loops are disposed back-to-back in overlying relation to the upper surface of said core tongue and said second and fourth shield loops are disposed back-to-back and in underlying relation to the lower surface of said core tongue. 